The separation columns commonly used in liquid chromatography and supercritical fluid chromatography consist essentially of a tube filled with solid particles forming the stationary phase. This packed column is equipped at both ends with special fittings, via which it can be connected to the analytical measuring instrument, such as the chromatograph. During operation, such columns are typically subjected to high pressures of up to 400 bar.
Current separation columns for chromatography usually consist of a stainless steel tube in which the stationary phase is packed. The stationary phase is retained at each end of the stainless steel tube by a sieve or frit. Furthermore, fittings, typically also of stainless steel, are provided at the ends. An example of a separation column in the art is disclosed in U.S. Pat. No. 4,737,284.
In addition to the stationary phase, the surface quality of the tube inside wall is especially important for the chromatographic behavior and service life of the separation column. If the surface is not sufficiently smooth and homogeneous, this adversely affects the flow behavior of the sample substances undergoing separation by passing through the column. In particular, variations in local flow rates can occur in the tube, causing asymmetry in the chromatographic peaks. As a result, quantitative analysis is impeded or subjected to errors. Due to the stringent requirements placed on the surface quality of the tube inside wall, conventional chromatographic tubes are expensive.
An additional limitation of stainless steel columns arises under extreme conditions with respect to pH and the type of mobile phase. Polymer stationary phases are now available that permit chromatographic separations across the entire pH range from strongly acidic (&lt;pH 3) to strongly basic (pH 12-13). Varying the pH range permits, for example, acidic or basic substances to be transformed to their conjugate molecular structure and then analyzed using reversed-phase chromatography as a neutral molecule without further additive in the mobile phase. Although polymer phases, in contrast to silica gel phases, have extended the working range by allowing the use of strongly acidic/basic mobile phases, the corrosive action on stainless steel tubes under these conditions remains a disadvantage in practical use.
As the sample substances increase in complexity, increasingly complex separation methods/conditions are required; stainless steel columns are not always satisfactory under these conditions, such as under high concentrations of oxidating salts in the mobile phase.
Moreover, problems can arise with stainless steel columns if there are samples to be analyzed that interact chemically with the stainless steel. The stainless steel material, which, as is well known, is manufactured from a melt of different metal ions, can act as a Lewis acid, which can form metal chelates with certain sample substances such as b-diketones, dihydroxy-naphthalene compounds, or various heterocyclic compounds.
In addition to stainless steel, other tube materials have proven themselves, primarily for bioanalytical applications. Such materials are:
stainless steel with a glass-coated inner surface, PA1 polyetheretherketone (PEEK), PA1 polytetrafluoroethylene (PTFE), and PA1 borosilicate glass.
For example, DE-A-2 930 962 discloses a separation column with a glass tube. The cited tubes do not resolve all problems occurring with stainless steel columns, however. Furthermore, they fail to offer the special advantages of stainless steel tubes. In particular, they lack pressure stability under the high pressures common in chromatography of up to 400 bar during chromatographic analysis and up to 1000 bar during the packing process. Moreover, glass is fragile and plastic can swell when using certain mobile phases such as tetrahydrofuran (THF).